Order disorder in sillimanite |
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| Authors: | S K Saxena |
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| Institution: | 1. Department of Geology, Brooklyn College, City University of New York, 11210, Brooklyn, N.Y.
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| Abstract: | AP3+-Si4+ disordering in sillimanite cannot be considered as ideal mixing on tetrahedral sites. Ideal free energy of mixing in sillimanite corresponding to slight disordering results in changing the kyanite-sillimanite phase equilibrium boundary by impossible magnitude. A regular solution model for the sites yields the following expression for the free energy of mixing: 1 $$G_{sill}^M = W \{ (X_{Al - T1} )^2 + (X_{Al - T2} )^2 \} + RT ln X_{Al - T1} X_{Al - T2}$$ . Calculations using the phase equilibrium data show that even small disequilibrium in intra-crystalline distribution results in significant change in pressure of the kyanite-sillimanite “equilibrium”. If information on interaction energies such as W is not available for disordering silicates, it is not advisable to use the ideal configurational entropy in calculations of phase equilibria. |
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